Clemens, in U.S. Pat. No. 3,842,194; 3,842,217; and 3,909,517, has described a conductive video disc comprising a molded plastic disc having video and audio information in the form of geometric variations in a spiral groove in the disc surface. These discs are coated first with a conductive material, such as a metal, which acts as a first electrode and then with a dielectric layer, such as an inert polymer layer. A metal-tipped stylus acts as a second electrode of a capacitor and the information signals are monitored by the stylus which notes changes in capacitance between the stylus and the disc surface as the information, in the form of depressions, passes beneath the stylus when relative motion is established between the disc and the stylus.
Further developments in this system have produced a video disc which is made of a conductive plastic material, such as a polyvinyl chloride resin containing sufficient amounts of conductive particles, e.g., carbon black, so that the disc can provide capacitance readout. The conductive particles are surrounded by organic material such as the plastic resin, lubricants, and the like, which provide a dielectric surface layer on the conductive particles. This development has eliminated the need for separate coatings of metal and dielectric on the plastic disc and thus has greatly improved the economics of the system.
In other developments, video discs which do not require a grooved surface have also been developed. With this type of disc the stylus is maintained in synchronization with an information pattern by means of electrical signals on either side of the information track, rather than physically by means of the groove walls.
The carbon loaded PVC molding compositions known to date are unsatisfactory in certain respects. One such example of this is that with the addition of large quantities of conductive carbon particles to a molding composition the melt viscosity of the molten molding mixture is greatly increased, which in turn increases thebrittleness of the disc molded therefrom. In order to obtain uniform disc replicas having very small and precise information patterns, large quantitites of plasticizers and processing aids have been added to the molding composition. This, unfortunately, also contributes to a low heat distortion temperature of the resin and makes it dimensionally unstable in hot weather, leading to rippling and shrinking which renders the disc unplayable. Thus, research has been continuing to improve the processability of the carbon loaded molding composition, while increasing the heat distortion temperature and the mechanical as well as the electrical properties of the discs molded from these compositions.